With baby Vanessa, I thought in that case that taking the baby but leaving the mom's body might indicate the baby was the target of someone who wanted a live baby and was willing to kill to get one.
I find it a lot harder to see that being the case here.
And, as everyone's already pointed out, how far would the mom of a 2-week old baby walk from home without her purse or some things for the baby? The clothing Heidi was wearing at the school in the morning seemed pretty distinctive, and I think a lot of people notice a person with such a tiny, new, baby. So I would have thought someone would remember seeing her if she was walking around.
Has it been stated what floor Heidi lives on? Would she go down in an elevator? Could she have popped downstairs to take some trash out and not locked the door behind her, and then been attacked? If she had been attacked like that, I would expect it to be some kind of mugging, not a kidnap of a young mum and very young baby too.